Method and apparatus for electroplating rollable objects



M. A. HUNTER 3,386,895

METHOD AND APPARATUS FOR ELECTROPLATING ROLLABLE OBJECTS June 4, 1968 Filed Nov. 27, 1964 FIG. 1

FIG. 2

INVENTOR.

MEREDITH A. HUNTER p4 5 BY an. 4452 United States Patent 3,386,895 METHOD AND APPARATUS FOR ELECTRO- PLATING ROLLABLE OBJECTS Meredith A. Hunter, Lexington, Ky., assignor to International Business Machines Corporation, New York, N .Y.,

a corporation of New York Filed Nov. 27, 1964, Ser. No. 414,310 7 Claims. (Cl. 204-23) ABSTRACT OF THE DISCLOSURE Rollable objects are electroplated in an apparatus comprising a non-magnetic electrically conductive tank acting as cathode and including an anode for passing a plating current from the anode to the cathode and having means fixedly surrounding the tank for inducing a high speed rotating magnetic field through the tank. The magnetic field causes rolla'ble objects within the tank to move around the walls of the tank and thereby become plated. The magnetic field means comprise a plural phase, plural oil electrical circuit.

This invention relates to an apparatus and process to position objects for efiicient plating or coating from a bath under the influence of electric current. Although the invention is directly concerned with coating of the electroplating type, the means provided have to do with positioning and are not related directly to the chemical aspects of the coating process.

Electroplating of parts as performed in the prior art is often both excessive in cost and far less than ideal in result. To plate an object with an electric current the object must be made an electrode in an electrolyte containing circuit carrying electric current. The electric circuit includes the object, a plating bath, and a second electrode. As is well known, electric current passing between the electrodes separates elements from the plating bath. A multitude of variations is possible. Generally, metal ions are attracted from the plating bath to the object where they are neutralized to form a thin coating of the metal on the object.

An inherent problem in this plating is in connecting the object as an electrode. Permanent structures are easily provided to contain the plating bath, to position one electrode in the bath, and to present an energized electrical circuit to the system. A plurality of objects to be plated, however, each must be brought to the system, connected as the second electrode, and then removed after being plated.

In certain applications the objects can simply be vigorously agitated and mixed while they are in the plating bath. The tank for the bath then often takes the form of a horizontally disposed barrel, the inside of which is conductive and in the energized electrical circuit. A large number of objects to be plated are deposited in the barrel, and the barrel is rotated. The objects are carried up the side of the barrel until they fall back under the influence of gravity. Each object is masked by other objects and the sides of the barrel during part of the time. The mixing action of the moving barrel results, however, in many different positions being assumed and all of each object ultimately being exposed and plated.

A vigorously agitated system inherently includes discontinuities in the plating process. As objects fall or are moved away from the sides of the tank, they are no longer electrodes in the electroplating system, although they become electrodes once more when they again come in direct electrical contact with the sides of the tank. Some materials plate satisfactorily in a barrel system regardless of the discontinuities of current flow in the plating process. Other materials, an example of which is nickel, undergo an undesirable chemical change during the time that plating is discontinued. When nickel is plated, a nickel oxide film tends to form during discontinuities in plating. Further plating is on the external surface of an oxide film, and the resulting product is a laminated coating with poor adhesion between the layers of nickel.

Chromium can be plated in a barrel system without the resulting product being laminated. Barrel plating of chromium is not commercially practical, however, because of considerations having to do with the electrical system. Chromium ions in a plating bath are often in a hexavalent state of ionization. This requires that six electrons must be supplied by the electrical circuit to deposit a single chromium atom. Considering valence alone a ratio can be computed of atoms plated to current supplied for plating. For chromium this ratio attributable to valence is one-third of that for most other metals because most other metals are bivalent. A second factor drastically reduces the efficiency of chromium plating. This is the so-called cathode current efiiciency. The cathode current efficiency is a measure of the amount of useful plating current as compared to current consumed in liberating by-products such as hydrogen gas. Cathode current efficiency is often only 17% for chromium plating as compared to -100% for the plating of many other types of metal deposits.

In a barrel or other agitation system the objects being plated tend to settle in a pile. Although agitation is con- 7 ducted, most of the objects are masked to a great extent by other objects much of the time when they are in contact as electrodes. It is often impractical to plate chromium in such a system. Relatively long periods of high intensity electric current accompanied with physical agitation of the objects is required. Not only is the equipment time consumed excessive, but the heat generated by the large current must be dissipated by the use of expensive structures and arrangements.

In the prior art, therefore, certain electrical coating processes require that a large portion of the object be continuously connected as an electrode during the entire plating process. Each object had to be brought to the system, semi-permanently connected as the second electrode, and then removed after plating. In the prior art the positioning of the objects is generally done by hand, fven when the number of objects to be plated was very arge.

Small, circular metallic washers are often chromium plated by prior art techniques. Each washer is picked up by a workman and connected by pressure contact with a clip at the internal opening found in washers. The clip is conductive and is connected to the permanent electrical source. The washer held by the clip is then dipped into the bath by automatic mechanical means. Since the washer is conductive, it becomes a semipermanent electrode in the system. Plating can be immediately brought about under the influence of the electrical circuit provided.

Although a plurality of clips can be used to simultaneously plate a plurality of objects, the highly undesirable requirement to handle each object remains. Furthermore, a static connection between the clip and the object is made at one point. No plating is accomplished at this point because it is covered by the clip.

This invention dispenses with the clip and furthermore eliminates any need to manually attach each object to the clip or to any other point. This invention uses magnetic force to position objects properly as electrodes, and this invention is believed to be completely novel in its use of magnetism.

It is an object of this invention to largely eliminate manual steps in the electrical plating of one or a plurality of items.

It is a further object of this invention to provide a system to electrically plate items at greatly reduced cost.

It is another object of this invention to position one or a plurality of items in an electrical circuit automatically in response to the single closing of a switch.

It is an additional object of this invention to provide an automatic positioning system for items in an electrical plating system in which the electrical contact position is altered so that the entire item will be plated without interruption of the plating current.

It is a more specific object of this invention to provide a system to position and plate with chromium a large number of objects in a few minutes.

In accordance with the invention a magnetic field is provided which passes through a tank holding an electrical plating bath. One electrode may be permanently fixed within the bath. At least portions of the walls of the tank are suitably conductive to provide a contact such that a conductive object resting against the side of the tank will act as an electrode in an electrical plating system. The magnetic forces draw objects to be plated to the side of the tank. An electrical voltage between the one electrode and the conductive tank walls passes through the objects and results in plating of the known type from a plating bath within the tank.

In accordance with further aspects of the invention, the magnetic field provided moves at least intermittently. In the preferred embodiment a field rotating past the vertical walls of the tank is provided by a three phase, three coil alternating current system similar to that found in many electrical motors. The moving field positions the objects and also tends to carry the objects in the tank with it somewhat. The objects are positioned side by side. A torque is induced upon the objects and they therefore tend to roll along the side of the tank. As the objects roll, different contact points are continuously made so that prior contact points are then exposed and will be plated.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a plain view of the invention showing washers positioned by the magnetic force and being rolled along the tank.

FIG. 2 is a cross-sectional elevation view showing the system of the invention.

THE ELECTROPLATING SYSTEM For purposes of explaining this invention it need only be emphasized that the electroplating system is conventional in every respect. In one actual use iron washers of the type shown in FIG. 1 and having an outside diameter of about /.i inch, an inside diameter of about inch, and a thickness of about inch were plated with chromium. The type of plating is clearly immaterial to the practicing of this invention, except that a ferromagnetic plating material such as nickel may plate out on the tank to create a possible shunt of the magnetic circuits described below for which compensations in the structures must be made.

Plating solutions are generally ionic and always conductive of electrical current. A heavy direct electrical current normally flows between the object being plated to the other electrode through the plating solution. Neither of these characteristics, however, substantially impairs the action of magnetic flux directed through the tank and plating bath. This is true because an ionic solution, a1- though electrically conductive, presents neither a shunt nor a substantial bar to magnetic fiux. The heavy direct current used has no unusual characteristics which would cause an interaction with a magnetic flux passing through it.

The system of the invention therefore consists of a tank 1 holding an electroplating bath 3. An electrode 5 extends vertically into the electroplating bath 3 at the center of the tank 1. The inner Wall 7 of the tank 1 is made of an electrically conductive, substantially non-ferromagnetic material such as stainless steel of a substantially non-ferromagnetic type. The inner wall 7 and the electrode 5 are connected through a switch 9 to a source of direct current electrical potential V.

The inner wall 7 is made of stainless steel because that material is relatively unreceptive to being plated upon. Plating of conductive portions of the tank, such as the inner wall 7, is a known disadvantage in this art. The plating can be removed at intervals with a strong acid or similar techniques.

The bottom of the tank 1 may be perforated (FIG. 2). A pipe 11 is situated over and near the center of tank 1. Pump 13 connects pipe 11 to the contents of a larger tub 15, which can be called the circulating tub 15. Circulating tub 15 holds relatively large quantities of plating bath solution 3. As will be clear subsequently, the preferred embodiment includes a moving magnetic field which can induce current in the objects being plated and in other conductive portions of the system. High direct currents are also used during the plating step. Circulation of plating solution 3 by pump 13 through pipe 11 and perforations in the tank 1 cools the electroplating system to prevent overheating. Circulation also refreshes the plating bath 3 with plating material at those points near the objects being plated which have been coated by the nearby portions of the bath 3.

THE MAGNETIC SYSTEM The magnetic system can be the three coil, three phase system found in three phase synchronous and induction motors. As a matter of fact, the stator system from a three phase induction motor has been used in a test model of the invention. The flux induced was entirely satisfactory, but the heat produced was a problem.

For purposes of description three sets of coils are shown, along with the voltage source associated with each. It should be understood, of course, that the magnetic system is isolated from the bath 3 by a jacket (shown symbolically in FIG. 1 by dotted outlines). As is well understood in the magnetic arts, three fiux patterns displaced degrees both in phase and in angular position add vectorially to create a single magnetic field rotating with an angular velocity proportional to the frequency of the electrical signals. A flux pattern more nearly corresponding to the theoretically perfect vector addition can be achieved when the number of magnetic poles is increased, each pole still being different in position and phase in accordance with the basic three pole, three phase system. Preferably, a magnetic system with more poles than the basic three will be used with the invention to obtain a well defined, moving magnetic field.

With reference to FIG. 1, it will be seen that the magnetic system consists of one set of coils 17' and 17", oppositely faced and wound in the same direction. Coils 17 and 17" are in circuit with a source of alternating voltage 19, having a relative phase of 0. Coils 17' and 17" are connected by a conductor 21 (shown in dotted outline because it is hidden by tank 1). The fiux created by coils 17 and 17" is directed across a diameter of tank 1 and the flux of the two coils 17' and 17" is additive.

Coils 23 and 23" are in a circuit similar in every respect to the circuit of coils 17' and 17". The coils 23 and 23" are displaced angularly 120. Their potential source 25 leads in phase by a relative 120. Conductor 27 connects the pole faces.

In the same manner, coils 29' and 29" are displaced a further 120 angular degrees and are powered by voltage source 31 which leads in phase a further 120 degrees. Conductor 33 connects the two pole faces.

It will be noted as suggested in FIG. 2 that the coils 17, 17", 23', 23 and 29, 29" are preferably slightly larger than the vertical dimension of the tank 1. This is to assure that all objects in the tank are operated upon by magnetic flux. A switch 34 is provided for control of the magnetic system. The switch controls all of the electrical circuits to the coils simultaneously. If the tank 1 is made of thick metal, it should be laminated to reduce eddy currents. The outside of tank 1 need not be conductive. Therefore, it is desirable to protect this side from the plating bath with an inert, plastic layer.

OPERATION OF THE INVENTION (A) Sequence To practice the invention washers 40 or similar objects are simply dumped into the tank 1. There is no need to handle each washer separately. They may be piled randomly on the bottom of the tank 1. Switch 34 may be open at this time so that no magnetic elfects are present.

Switch 34 is then closed. The magnetic field operates on each washer 40. The washers are pulled to the side of the tank 1 as shown in FIG. 1 and FIG. 2. Magnetism is induced into each washer. The poles of magnetism induced in each washer apparently attract opposite poles in' the other washers. The washers tend to line up side by side as shown in FIG. 1. The major magnetic force is from the magnetic system and the washers therefore are pulled and held to the side of tank 1.

' The washers roll gently under the influence of the rotating magnetic field created by the magnetic system as described. Various points on the washers are thus .presented for electroplating. The side-by-side configuration assumed is significant also since this configuration leaves most of the washers exposed for plating. The unexposed portions subsequently are presented as the washers roll along the side of the tank. No break in the plating process occurs, this being highly significant in plating certain materials which tend to laminate in the manner discussed with reference particularly to nickel.

Electroplating is conducted while the washers 40 are magnetically positioned. Closing of switch 9 is all that is required to cause plating, since the proper circuit through the plating bath 3 is inherent in the structures. Pump 13 circulates bath 3 for cooling purposes in the manner previously discussed. A single batch of washers is plated with chromium sufiicient'for commercial utilization in a time period on the order of magnitude of 1 minute to 45 seconds as compared to minutes to 30 minutes to perform the same plating in a barrel system. The exact time varies, of course, with the exact conditions and components of the system.

(B) Theory of operation The attracting of the metal objects and their lining up side by side has been discussed above. The edges are presented to the magnetic field produced by the magnetic system. This is in response to the usual tendency of magnetic flux to travel through the most receptive magnetic path. Other configurations are adopted for relatively heavy objects, the longest axis of which apparently can not be held by the magnetic forces in an extended position.

The rolling or rocking of the washers can be explained by at least two, complementary theories. It will be recalled that the magnetic system effectively induces a single magnetic field rotating at high speed around the tank. The velocity of this field is proportional to the radial distance from the center of rotation. It will be clear that the washer will'act as rotors in a small induction motor because the outer side of the washer is intersected by flux moving at greater speed than the same flux as it intersects the inner side of the washer. As in any motor, a torque will be induced tending to cause the rotor to follow the moving flux.

A second factor would be the residual magnetism which tends to stay with the objects after the magnetic field which induced the magnetism has moved. Thus, the flux from the magnetic system induces magnetism directly across the washer. The flux then moves, leaving the magnetism previously induced in the washer somewhat as it was. A torque will be created by the new relative positions, with the flux pulling the original outside portion of the washer (the portion in contact with tank 1) toward it, and pushing the inside portion of the washer away.

In the invention the washers do not move quickly. Instead they roll gently, even stopping at times. The washers might be expected to fall during those periods when the rotating magnetic field has moved from them. A 60 cycle alternating current is used in the invention, however, to produce a magnetic field making 60 complete revolutions each second. Inertia in the system apparently prevents the washers from falling away during the short time which they theoretically could. Instead they extend continuallyy from the side of the tank while rolling gently.

No unobvious limitation is known concerning the objects being plated. They must be of a nature to be moved to and be held by a rotating magnetic field. In practicing the preferred form of the invention the objects plated should be rollable, i.e., conformed to turn under the influence of a torque so as to continuously present different contact positions to the tank.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Iclaim:

1. An electrical plating apparatus comprising:

a tank for containing an electrical plating bath, said tank having continuous side walls, the inside of said side walls being composed of conductive substantially non-ferromagnetic material,

plural phase, plural coil electrical circuit means fixedly situated across and outside of said side walls for electrically inducing a single magnetic field rotating at least intermittently through said tank, and

an electrode situated within said tank for electrical plating between said electrode and objects in contact with said side walls.

2. The apparatus as in claim 1 wherein the means to induce a magnetic field induces a magnetic field rotating normally repetitively completely around said side walls.

3. The process of electrical plating comprising the steps of:

placing a plurality of rollable objects in a tank containing a plating bath, said tank having side walls at least partially conductive,

electrically inducing a single magnetic field in said tank by a plural phase, plural coil electrical circuit means fixedly situated across and outside of said side walls, said field being sufficiently strong to pick up said objects and move said objects into contact with said side walls above the bottom of said tank and thereafter, rotating said magnetic field at least intermit tently to cause said objects to roll along said walls while,

inducing an electrical current through conductive portions of said side walls to electrically plate said objects in contact with conductive portions of said side walls from said plating bath.

4. The process as in claim 3 wherein said magnetic field is a rotating magnetic field moving at high speed around said tank.

5. The process as in claim 3 wherein said magnetic field is a rotating magnetic field rotated at 60 complete revolutions each second.

6. A process of electrical plating comprising the steps of:

placing a plurality of rollable objects in a tank containing a plating bath, said tank having continuous side walls, the inside of said side walls being composed of conductive, substantially non-ferromagnetic material,

electrically inducing a single magnetic field through said tank across said side walls by a plural phase, plural coil electrical circuit means fixedly situated across and outside of said side Walls said field being sufficiently strong to pick up said objects and move said objects into contact with said side walls above the bottom of said tank and thereafter, rotating said magnetic field at least intermittently to cause said objects to roll along said tank while,

inducing an electrical current through said inside of said side walls to electrically plate said objects in contact with said inside of said side walls from said plating bath.

7. The process as in claim 6 wherein said magnetic field is a rotating magnetic field rotating at 60 complete revolutions each second.

References Cited UNITED JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

